1. Field of the Invention
This invention is directed to a method of forming glasses without blisters in manufacturing processes containing refractory metal systems, and in particular, refractory metal conditioning systems. The invention is particularly useful for, but not limited to, forming high melting or high strain point glasses, such as are used for glass substrates for flat panel display devices, and glasses which are essentially arsenic or antimony-free, in manufacturing processes utilizing refractory metals, such as platinum or platinum alloys, which contact the glass.
2. Technical Background
Manufacture of hard alumino-borosilicate glass, e.g. for flat panel displays, requires low levels of gaseous inclusions (blisters). Advantageously, refractory metal conditioning systems may be used to avoid refractory outgassing in the final stages of melting, where any blister formed will become a defect. Such refractory metal systems (typically Pt or Pt—Rh alloys) are generally considered to be inert in relation to most glasses, and thus not cause any inclusions in the final glass product. However, this is not necessarily valid. Reactions that occur at the metal/glass interface inside the vessels lead to the generation of gaseous inclusion within the glass melt and thus the final glass product (e.g. glass sheet).
One of the more common reactions that occur at the metal/glass interface is the conversion of negatively charged oxygen ions to molecular oxygen which is caused by the thermal breakdown of water and hydroxyl (OH) species in the glass melt. This phenomenon occurs because at the elevated temperatures of glass melting and delivery, a low partial pressure of hydrogen exists in the glass melt. When hydrogen in the glass melt comes in contact with the refractory metal vessel, the hydrogen rapidly permeates out of the vessel and into the atmosphere outside the vessel, depleting the refractory metal/glass interfacial region of the glass melt of hydrogen. Thus, the permeation of hydrogen from within the vessel, through the vessel wall(s) into the atmosphere outside of the vessel increases the free oxygen within the molten glass (glass melt). Thus, in the breakdown of water, for example, for every two moles of hydrogen that leaves the glass melt due to permeation, a mole of oxygen is left behind at the metal/glass interfacial region of the melt. As the hydrogen leaves the glass melt, the oxygen (or the partial pressure of oxygen) increases, leading to the generation of blisters or gaseous inclusions within the melt.
The hydrogen permeation rate through the walls of the refractory metal vessel, and, therefore, the generation rate of neutral molecular oxygen at the refractory metal/glass melt interface, is proportional to the difference between the square roots of the external and internal hydrogen partial pressures.
Additionally, there are other reactions which involve the catalyzing or oxidation of other species in the glass melt, such as halogens (Cl, F, Br), that can lead to the generation of gaseous inclusions. Further, electrochemical reactions can occur at the metal/glass interface. These electrochemical reactions can be associated with thermal cells, galvanic cells, high AC or DC current applications and grounding situations.
For some applications, blisters in the glass drawn from the glass manufacturing system may pose little problem, and may, under some circumstances, provide aesthetic value. However, in the manufacture of glass substrates for use in the manufacture of flat panel display devices such as liquid crystal displays (LCD) and organic light emitting diode (OLED) displays, blistering makes the resultant glass substrate (sheet) unusable. Therefore, there is tremendous advantage to a process which mitigates against the presence of gaseous inclusions.
Today, there are several known methods that can be used to address the foregoing reactions which cause the formation of blisters in the glass melt. One known method that can be used to help minimize the formation of gaseous inclusions in the finished glass product involves the use of fining agents in the glass melting and conditioning stages. Fining agents are multivalent oxygen-containing compounds that release or absorb oxygen based on temperature.
Arsenic is among the highest temperature fining agents known, and, when added to the molten glass batch material, it allows for O2 release (reduction) from the glass melt at high melting temperatures (e.g. above 1450° C.). This high temperature O2 release, which aids in the removal of gaseous inclusions during the melting and fining stages of glass production results in a glass product (e.g. glass sheet), that is essentially free of gaseous inclusions. Furthermore, any residual oxygen is reabsorbed by the fining agent during the transition from a reduced state to an oxidized state as the glass cools.
Unfortunately, from an environmental point of view, arsenic is considered a hazardous material, and therefore undesirable as a fining agent. Other, less effective fining agents are available, such as antimony and tin, but antimony is also environmentally undesirable, and tin much less effective than either arsenic or antimony.
The need to eliminate environmentally unfavorable but effective fining agents, such as arsenic and antimony, in order to remove gaseous inclusions in the glass melt has required the use of less effective fining agents, and placed greater emphasis on addressing the generation of gaseous inclusions.
There are other methods available to mitigate reactions that lead to the formation of gaseous inclusions in the glass. U.S. Pat. No. 5,785,726, for example, discloses a humidity controlled enclosure that surrounds one or more platinum-containing vessels and is used to control the partial pressure of hydrogen outside the vessel(s) so as to reduce the formation of gaseous inclusions. The breakdown of the water in the moisture-laden atmosphere within the enclosure produces hydrogen which in turn helps to suppress the aforementioned hydrogen permeation. Although the enclosure described in U.S. Pat. No. 5,785,726 successfully reduces the formation of gaseous inclusions, it has several drawbacks. First, the enclosure is so large that it makes it difficult if not impossible to maintain a uniform environment around the platinum-containing components of the glass making apparatus. Second, the environment within the enclosure is so hot and humid that it can be uncomfortable to people that must work within the enclosure. U.S. patent application Ser. No. 11/116,669 improved upon the traditional enclosure described in U.S. Pat. No. 5,785,726, disclosing encapsulating the refractory metal components of a glass making apparatus within a relatively tight fitting enclosure (capsule). Use of a capsule allows improved control of the atmosphere within the relatively small volume between the capsule and the encapsulated glass-carrying refractory metal components. This is due to the fact that a probe reading (such as relative humidity or dew point temperature) for conditions inside the capsule is more likely to be representative of conditions at the exterior surfaces of the refractory metal glass processing equipment than measurements taken within the large volume of space within the prior large, room-sized enclosure. Additionally, if there is a process instability or change in the water content of the molten glass within the refractory metal vessel(s) that leads to an increase in hydrogen permeation blistering, then there is often no way to respond to this problem using the conventional enclosure disclosed in U.S. Pat. No. 5,785,726 since it may be operating at its maximum dew point. Moreover, response time of the capsule of U.S. patent application Ser. No. 11/116,669 to process instabilities is greatly enhanced owing to the small volume contained between the capsule and the refractory metal vessel(s) compared to the conventional enclosure. That is, the changes in the humidity (dew point) within the capsule volume, and therefore the hydrogen partial pressure, can be performed much more rapidly than would be possible in a room-sized enclosure.
Nevertheless, in spite of the improvements in hydrogen permeation blister control represented by the methods described above, they are based entirely on the water content of the glass, and control of the moisture content of the atmosphere surrounding the refractory metal vessels comprising the glass manufacturing apparatus. Moreover, these methods have to date been empirical in nature, and therefore applied with a large dose of guesswork with regard to the partial pressure of hydrogen necessary to suppress blister formation. It would be beneficial if hydrogen permeation blistering control could be undertaken with a more complete understanding of the impact of other blistering factors, such as the total concentration of multivalent oxygen-containing compounds within the glass. This has become particularly important as, for environmental reasons; the total concentration of multivalent compounds introduced into the glass batch has been decreased.